If you buy a second-hand car, you still need to check that the 'modifications' are acceptable to your insurer. Photograph: Alamy

Fitting a roof rack to your car or putting some stickers on its doors can seem a fairly innocuous activity, but these “modifications” can ring alarm bells with insurers.

“ ‘Modifications’ is a word greeted with caution by some insurers,” says Ian Crowder of the AA. From their perspective, any changes you make to your vehicle could be seen to increase its value or boost its potential theft risk, so your insurer may want alterations detailed on your policy and could even increase your premium accordingly.

If you fail to declare any changes you risk your claim being rejected in the event of an accident and “failing to disclose any modifications when applying for insurance could count as fraud”, warns Kevin Pratt of moneysupermarket.com

What counts as modifications?

“If you’ve altered your car in any way since it left the factory or showroom, this can be deemed an after market modification,” says Matt Oliver from GoCompare.com. And if you’re buying a vehicle second-hand you still need to declare any modifications that have been made by the previous owner. “The fact you didn’t make them might not be taken into account by your insurer,” he says.

Modifications tend to fall into two categories, “power related” or “cosmetic”, according to a Direct Line spokesperson. Changing the engine size or fitting alloy wheels sound like obvious power-related changes that you should tell your insurer about; but when it comes to cosmetic modifications it can be a grey area. Cosmetic changes can cover anything from “stickers and slogans to tinted windows, parking sensors, tow-bars and spoilers”, Crowder advises.

However insurers set their own rules. At Direct Line, stickers on a vehicle would count as a cosmetic modification and be judged on “an individual basis”, whereas other insurers such as LV= say stickers don’t count. Even the humble roof rack could incur the wrath of your insurer if you don’t declare it. It’s just one of a list of modifications the Post Office, for example, will want to know about, along with changes to the internal upholstery, parking sensors, tow-bars and stickers, badges and murals.

While switching insurers can save money it’s worth remembering that they may take a different stance on modifications. “Just because your last insurer was fine with any modifications you made to your vehicle doesn’t necessarily mean your next one will be,” Oliver says.

Insurance applications can be refused and “while fewer than one fifth may be turned down, of those that are around a half would relate to modifications”, Crowder says.

The sticker factor

If stickers can count as a modification, does this mean you should be on the phone to your insurer every time you pop one on the windscreen to advertise your local radio station?

Crowder says “having a sticker saying you’re a member of the National Trust, or a ‘baby on board’ sticker really shouldn’t make a difference”. However it’s still worth checking.

Changes once your policy’s active

If you’re planning to make changes to your car don’t just assume it’s OK to go ahead. Small print in Direct Line’s motor policy says: “You must tell us what modifications you intend to make and obtain our agreement prior to making them.”

It’s the same with Admiral, which expects customers to tell it before they modify their car, including any cosmetic changes and resprays.

If you go ahead without the company’s agreement it could be within its rights to withdraw your insurance cover, so always ask before making any modifications.

Hidden cost of customising your car

If you’re organised enough to ensure you carry out any changes at policy renewal you could save money, as most insurers impose “mid-term adjustment fees which can be between £25 to £30,” according to Pratt.

Relatively minor modifications could prove costly in admin fees alone (if your insurer decides they should be listed on your policy) even if the premium stays the same. However, if on renewal you let your insurer know of any imminent modification change it may waive the additional admin fee.

And if any of those bumper stickers, vinyl wraps, or murals on your vehicle are there to promote your business check your insurance covers you for commercial purposes, too.